1. Field of the Invention
The present invention relates generally to the field of vacuum deposition processes, and more particularly to a linear aperture deposition apparatus and coating process for coating wide substrate materials.
2. Relevant Technology
Optical interference coatings are useful for controlling the reflection, transmission and/or absorption of a selected wavelength range of light. These coatings consist of a plurality of alternating layers having a predetermined thickness less than the selected wavelength range. Additionally, the layers have a significant difference in refractive index and are controlled to a predetermined thickness. Suitable materials for optical interference coatings are primarily dielectric materials which have a refractive index range of about 1.4 to about 2.4, which is wavelength dependent, and a very small optical absorption coefficient. In some applications, thin layers of metal films, which have large absorption coefficients, are combined with the dielectric material layers.
The economical production of these coatings is frequently limited by the thickness uniformity necessary for the product, the number of layers, and the deposition rate of the coating materials. The most demanding applications generally require that the deposition occur in a vacuum chamber for precise control of the coating thickness and the optimum optical properties. The high capital cost of vacuum coating equipment necessitates a high throughput of coated area for large-scale commercial applications. The coated area per unit time is proportional to the coated substrate width and the vacuum deposition rate of the coating material.
A process that can utilize a large vacuum chamber has tremendous economic advantages. Vacuum coating chambers, substrate treating and handling equipment, and pumping capacity, increase in cost less than linearly with chamber size; therefore, the most economical process for a fixed deposition rate and coating design will utilize the largest substrate available. A larger substrate can generally be fabricated into discrete parts after the coating process is complete. In the case of products manufactured from a continuous web, the web is slit or sheet cut to either a final product dimension or a narrower web suitable for the subsequent manufacturing operations.
The manufacturing cost of the product is ultimately limited by the specific performance requirements that limit the maximum deposition rate. For example, if the required uniformity of coating on a continuous web or film is 1% or less over 12 inches of width, one would generally operate the source at the highest deposition rate, Rmax, that could consistently yield the requisite 1% uniformity over the 12-inch width. If operation at that deposition rate degraded another specified characteristic, such as the maximum defect size, below a minimum acceptable value, then the deposition rate would be lowered to R.sub.1, where R.sub.1 &lt;R.sub.max.
Continuing with this example, further cost reduction could be achieved if the coating were deposited on substrates having widths that are multiples of 12 inches; i.e., 24 inches, 36 inches, etc. For example, if a 36-inch-wide source achieved 1% uniformity at deposition rate R.sub.1, it would cost less to coat a 36-inch-wide substrate and slit it to a final width of 12 inches than to coat a 12-inch-wide substrate, because three times as much material would be produced by the wider coating machine. A wider coating machine would cost less than three times the cost of a 12-inch coating machine, perhaps only 50% more. However, this advantage would only be realized if the 36-inch source could deposit the coating with 1% uniformity over the entire 36-inch substrate width at a rate, R.sub.2, which is greater than or equal to R.sub.1, without exceeding the maximum defect size.
Therefore, in the case of continuous coating equipment, in which a substrate of a fixed width is transported over each source to deposit the coating design, simultaneously improving the uniformity of the source and the deposition rate without degrading the film properties, will have a profound economic benefit.
Two techniques are commonly used in the physical vapor deposition of coating materials. These are sputtering and thermal evaporation. Thermal evaporation readily takes place when a source material is heated in an open crucible within a vacuum chamber when a temperature is reached such that there is a sufficient vapor flux from the source for condensation on a cooler substrate. The source material can be heated indirectly by heating the crucible, or directly by a high current electron beam directed into the source material confined by the crucible.
Magnetron sputtering adapts well to coating wide substrates with metal layers. The length of the magnetron assembly is selected such that the sputtering racetrack exceeds the substrate width by several inches at each edge, wherein this central portion of the racetrack provides a uniformity in thickness that is typically less than about 5%. However, magnetron sputtering equipment is relatively expensive, is limited to materials that can be readily formed into solid targets, and has deposition rates that are generally inferior to those of thermal evaporation technologies, especially for metal compounds that are useful as optical coating materials.
A Knudsen cell evaporation source is an isothermal enclosure or crucible with a small orifice that confines the source material and requires the vapor to diffuse out of the orifice. The inside of the cell is large compared with the size of the orifice to maintain an equilibrium interior pressure.
The enclosed nature of the Knudsen cell reduces the likelihood that particulate ejected by the solid source material, commonly known as spatter, will reach the substrate either to cause damage or to be embedded therewithin. It is generally believed that such spatter is generated by the non-uniform heating of a granular or otherwise non-homogeneous source material whereby locally high pressures cause the ejection of the most friable portions of the source material. Spatter is severe in source materials with a low thermal conductivity and having retained moisture, air or other high vapor pressure components, and increases with the heating rate due to increased temperature differentials.
Thermal evaporation generally has been adapted to coating wide substrates by two methods. The most common method is to create a linear array of point sources, each point source being a small crucible having either a common or individual heating source. An alternative technique is to confine the source material in an elongated crucible and sweep an electron beam over the entire length of the crucible in order to uniformly heat the source material. A linear crucible must uniformly heat the coating material to achieve a uniform flux of coating material vapor across the entire substrate width.
The principle of a Knudsen cell has also been applied to coating wide areas. The cell enclosure is a tube or rectangle matching the width of the substrate and having a constricted slit along its entire length. Although a tubular Knudsen cell is easy to fabricate, it can be difficult to uniformly fill with solid source material, especially when the slit is relatively narrow with respect to the width of the source material particles. U.S. Pat. No. 5,167,984 to Melnyk et al., discloses further details optimizing a tubular Knudsen cell. The crucible has an open end suitable for alignment of a hollow cylindrical insert containing the source material. The source was designed and optimized for depositing chalcogenide photoconductive compounds and organic photoconductive materials.
U.S. Pat. No. 4,094,269 to Malinovski et al., discloses a tank-shaped source with a rectangular slot on its surface for the vapor deposition of silver halide compounds onto glass substrates and polyester substrates.
Prior art methods of depositing dielectric materials from either a series of electron beam point sources or linear crucibles have numerous limitations, especially for the economical production of optical interference coatings. They typically utilize less than about 15% of the source material evaporated, the balance of the source material being deposited on the coating chamber interior and masking fixtures. Both the chamber and masking fixtures must be cleaned periodically, resulting in lower utilization of the capital equipment capacity and higher material costs.
Masking fixtures are commonly used to correct for source non-uniformity in the direction transverse to the substrate's linear motion, a direction referred to herein as the "cross web direction". (The use of the term "cross web direction" is not meant to limit the present invention to plastic films or web products as the coated substrate.) The mask decreases the deposition rate further, to the minimum value along the source width.
Attempts to increase deposition rate by increasing source power input, such as electron beam current, result in either an unstable melt pool, or can further decrease the coating uniformity or increase the rate of particulate ejection, i.e., spatter, from solid or subliming and liquid materials. Either coating uniformity or surface quality considerations always limit the deposition rate.
The development of a linear source for the evaporation of higher refractive index materials has been a particularly elusive problem. While some successes have been obtained in depositing silicon monoxide and materials that sublime at a temperature less than about 900.degree. C., this limits the available refractive index to a range from about 1.6 to about 1.9.
Many of the more useful high index materials in optical coatings, such as titanium dioxide, zirconium dioxide and niobium pentoxide require heating to a much higher temperature to obtain the necessary vapor pressure for vacuum coating, typically from about 1800.degree. C. to greater than about 3500.degree. C.
There have been specific attempts to adapt forms of linear crucible sources to coating flexible plastic film in a continuous roll form. That is, the substrate is continuously unwound in the vacuum chamber to transport it over the evaporation source(s), the substrate being disposed around a large cooling drum, where it is brought into the desired spatial proximity to the linear crucible.
In U.S. Pat. No. 5,239,611 to Weinert, a crucible device is disclosed wherein a plurality of radiant heaters is disposed above the material to be evaporated. A series of outlets between the radiant heaters are in vapor communication with material being evaporated.
European Patent Application Nos. EP 0652303 and EP 0652302 to Baxter et al., disclose linear crucible evaporation sources. Referring to FIG. 1A, a prior art apparatus 20 is shown which corresponds to the evaporation source disclosed in the Baxter applications. The apparatus 20 has an evaporator 22 and a chilled drum 24 which transports a web substrate 26 to be coated across a deposition zone 28. The evaporator 22 includes a crucible 30, which is heated from below by a heating element 32. The crucible 30 is contained in a retort 34 having a lid 36, wherein lid 36 has a plurality of outlet nozzles 38 disposed in arcuate conformance to chilled drum 24. Referring to FIGS. 1B and 1C, outlet nozzles 38 may be a plurality of holes or narrow slots oriented in the substrate transport direction, i.e., perpendicular to the long axis of the source.
A linear evaporation source for use in web coating equipment is available commercially from General Vacuum Equipment Corp. of Birmingham, England. A cross-sectional diagram of this source is provided in FIGS. 2A and 2B. Referring to FIG. 2A, a coating apparatus 40 includes a drum 42 and a source 44. The source 44 includes a crucible 46 containing a source material 48. Vaporized source material travels from crucible 46 to a deposition zone 50 via a chimney 52. A fixed monolithic insert 54 is placed between source material 48 and chimney 52 at the top of crucible 46. An enlarged view of crucible 46, insert 54 and chimney 52 is shown in FIG. 2B.
Furthermore, prior art methods of coating plastic films are frequently limited to specific substrates depending on the heating load of the source and the substrate's heat deformation temperature. This limits the choice of coating materials that can be evaporated and the maximum coating thickness. The coating thickness (per pass by coating source) is limited in that a minimum web speed must be exceeded to avoid overheating the substrate.
Continuous vacuum coating of plastic substrates requires numerous compromises to be made in product cost, composition, performance or quality due to deposition source technology. There has been an especially acute need for an efficient thermal evaporation source for coating plastic films with high refractive index optical material, i.e., a refractive index greater than about 1.7, and preferably greater than about 1.9.
Zinc sulfide (ZnS) is a useful high refractive index optical material in both visible and infrared wavelengths. Its relatively low sublimation temperature range, from about 1000.degree. C. to about 1900.degree. C., would suggest that it is an ideal material for plastic web coating, but it has two inherent material problems. The deposition temperature must be well-controlled to minimize the decomposition of ZnS to zinc and sulfur atoms in the vapor state. Dissociation results in a sub-stoichiometric film, having an excess of zinc, when the zinc and sulfur recombine to form a solid film. Sub-stoichiometric ZnS has undesirable optical absorption. Also the uncontrolled dissociation results in residual sulfur compounds on vacuum chamber components, most notably in the vacuum oil, and an undesirable odor. Further, chemical reactions of the excess sulfur may accelerate the deterioration of various vacuum components.
Thus, there is a need for efficient linear evaporation sources that do not suffer from the foregoing disadvantages.